[opus] [RFC PATCHv1] cover: celt_pitch_xcorr: Introduce ARM neon intrinsics

[Viswanath Puttagunta]

On 25 November 2014 at 09:39, Jonathan Lennox <jonathan at vidyo.com> wrote:
>
> On Nov 25, 2014, at 10:07 AM, Viswanath Puttagunta <
viswanath.puttagunta at linaro.org> wrote:
>>
>> > Also is there plans to make the NEON optimisations on ARMv7 run time
>> > detectable like they have in cairo/pixman? For generic distributions
>> > it would nice to be able to be able to enable them as they offer
>> > decent performance improvements but have the code fall back on devices
>> > that don't support NEON.
>> Yep, adding support for ARMv8 is the final objective. I did not want to
introduce too many changes in the first shot... and hence only introduced
for ARMv7. In theory, most of the code (neon intrinsic code) in this patch
should remain unchanged for ARMv8. Only the mechanism by which neon/asimd
presence is detected during runtime and the flags used during compile are
the only ones that should change. I will work on this once this patch gets
reviewed and accepted. I made sure these changes are fairly localized.
>>
>> And yes, this patch also supports runtime detection of neon. Actually,
most of code to do run time detection of neon was already there in the
project before this patch. I just re-used the infrastructure.
>
> ARMv8 shouldn’t need Neon detection at all — Neon is a mandatory part of
the ARMv8 architecture, unlike ARMv7, where it’s optional.
As I understand, your statement for ARMv8 is true for AAarch64 mode. But
for ARMv8 in AAarch32 mode, neon is still optional (although I haven't
heard of an implementation that does not support NEON even in AAarch32
mode). So, for AArch64 mode, I think rtcd can be disabled. Also, even the
neon detection procedure currently in opus will not work on ARMv8 AArch32
mode. Please refer
http://community.arm.com/groups/android-community/blog/2014/10/10/runtime-detection-of-cpu-features-on-an-armv8-a-cpu
This is one of my todo lists after this patchset gets reviewed and accepted.

It looks like this is what the configure script is already doing — arm64
sets rtcd_support to no.
>
> I believe iOS, Windows RT/Windows Phone 8, and Blackberry 10 all require
CPU support for Neon when running on ARMv7+ platforms, so detection
shouldn’t be necessary there either.  The configure script should probably
default rtcd_support accordingly, but configuring with --disable-rtcd
should be sufficient to build on these platforms, regardless.
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